Non-emissive flat panel displays, such as LCD panels, are utilized to display images of textual and pictoral information to the viewer of a computer, overhead projector, television or the like screens. Typically an LCD panel comprises a pair of spaced, opposed glass plates or substrates between which an electro-optical material, in this case liquid crystal mixture, is disposed. Overlapping patterns of transparent, spaced electrodes on the respective substrates define picture elements or pixels at the areas in which they overlap. The optical state of each pixel, i.e., whether it appears bright or dark, is determined by the application of an electrical field across each of the areas at which the electrodes overlap. In high information content LCD panels the individual pixels are "addressed," i.e., the electrical field is applied to each pixel, by a display driving system.
The display contrast ratio of a typical LCD panel is defined as the ratio of the light transmitted through an area over many pixels in the bright state divided by the light transmitted through that same area when the pixels are in the dark state. In an LCD panel having a contrast ratio of less than about 10 to 1, the displayed image appears dull or washed out. At a contrast ratio of 20 to 1 or greater, the image appears sharper and better defined.
The transmission of light through the spaces between the electrodes and, thus, between each of the pixels, results in a diminution of the contrast ratio. For example, in a standard 10-inch diagonal LCD panel with 640 column electrodes overlapping 480 row electrodes to form 307,200 pixels in which the pixel pitch is 0.33 mm and the space between the pixels is 0.03 mm, the contrast ratio is typically about 8.3 to 1.
The cell gap, i.e., the thickness of the liquid crystal layer between the substrates of an LCD panel, and its uniformity are often critical to the quality of the display. Some attempts to block transmission of light through the spaces between pixels with relatively thick materials have been ineffective because they create non-uniformities in the cell gap. Some of the best masking materials are metallic which, when provided in relatively thin layers, have effective light-blocking characteristics. Direct application of such metallic materials, however, requires the imposition of insulating materials between the mask and the electrodes in order to prevent shorting of the latter, adding to the cost and complexity of the panel.
Placing a grid of mask materials on the outside of the LCD panel would introduce severe parallax effects and would be much less effective.